Using a radar to obtain information about distant, moving targets is practical in a variety of cases, including but not being limited to military surveillance, commercial airborne and seaborne navigation as well as scientific research. In this patent application we use the detection of space debris as an example. However, the same principles also apply to other kinds of radar measurements.
Space debris is a collective designation of all kinds of man-made orbital objects which no longer serve any useful purpose. Large (>10 cm) objects have known orbits and are routinely monitored by the U.S. Space Surveillance Network, but information of the smaller particles is fragmentary and mainly statistical. In order to remain in orbit an object must have a velocity of several kilometers per second in the Earth's coordinate system, which makes any such object potentially dangerous to satellites, manned spacecraft and other space-going vehicles. Exact information about space debris would help to plan space missions so that debris hazards could be minimized.
Ionospheric radars, such as the EISCAT (European Incoherent SCATter) radar system, exist that routinely perform radar measurements at altitudes that would also be of interest to space debris studies. However, these systems have been optimized for processing “soft” ionospheric reflections, and consequently are not well suited for measuring echoes from hard targets. As an example, a typical ionospheric reflection has a phase coherence time less than a millisecond, which is much shorter than the interval between consecutive transmitted pulses in a pulsed radar system (which for example in EISCAT is typically 3-10 ms). Therefore ionospheric echoes from individual pulses are uncorrelated and can only be added up in the power domain. Quite to the contrary, an echo from a hard target such as a debris object has a very long phase coherence time, in the order of several hundreds of milliseconds.
An approach known from the scientific report J. Markkanen, M. Lehtinen, A. Huuskonen, A. Väänänen: “Measurements of Small-Size Debris with Backscatter of Radio Waves”, Final Report, ESOC Contract No. 13945/99/D/CD, March 2002 is to utilize the transmissions of a pulsed ionospheric radar but to build a separate receiving and analysis system optimized for processing echoes from hard targets. FIG. 1 illustrates a radar system for collecting information about space debris following the model suggested in said report.
Radar transmissions are formed in a transmitter computer 101, amplified in an amplifier arrangement 102 and transmitted through an antenna 103. In a monostatic radar the same antenna 103 also receives the reflected signals. Bi- and multistatic radars are also known, in which reception takes place through different antenna(s) than transmission. In an RF receiver part 104 a preamplifier 105 amplifies the received signal and a mixer 106 converts it to a lower frequency. A detector part 107 comprises an A/D converter 108, a detector 109 and a buffer 110, from which buffered data is written to a temporary data storage 111. An analysis computer 112 comprises a scanner 113 and an analyzer 114, from which analysed data is taken to a final storage 115. The transmitter and receiver branches of the system operate in a common time base obtained e.g. from a GPS (Global Positioning System) receiver to enable time-stamping the measurement results. The transmitting branch may give control information, such as transmission waveform descriptions, to the receiving branch.
We assume that a debris object 120 proceeds along an orbit 121. A part 122 of said orbit happens to go through the radar's antenna beam 123. Parameters of interest, which the radar system should give as outputs of the measurement, are mainly range (distance between the antenna 103 and the object 120), radial velocity and radial acceleration of the object 120 as well as the signal amplitude (or signal total energy) of the reflection caused by the object 120. The last-mentioned could in optimal cases be used to estimate the size of the object. The scientific report mentioned above suggests that these could be obtained by using a mathematical method based on statistical inversion.
A problem of a measurement of the kind described above has previously been the vast amount of computing and data storage that is needed if measurements are to be made and results analysed in any reasonable time, or even real time. Some basic assumptions—coherent integration over 300 milliseconds, sampling interval 0.5 microseconds—give an input data vector having 600,000 points, an FFT (Fast Fourier Transform) of which requires about 60 Mflops (Mflop=million floating point operations). An estimated range requirement of 1000 km with a reasonable 1000/0.075≈13,000 range gates multiplies this to about 800 Gflops (Gflop=billion floating point operations). At the time of writing this description, an advanced workstation equipped with appropriate software and hardware means is capable of about 1 Gflop/s performance on FFTs of this length, which means that analyzing the measurement data of 0.3 seconds would take about 800 seconds of calculation.